1. Field of the Invention
This invention relates to processes for the production of methane from carbonaceous solids such as coal and is particularly concerned with gasification processes including a hydrogasification step in which hydrogen reacts with carbon to produce methane.
2. Description of the Prior Art
A serious decline in natural gas reserves has intensified efforts to develop processes for converting bituminous and lower rank coals into gas suitable for upgrading to high Btu synthetic natural gas. An important reaction in such processes is the strongly exothermic hydrogasification reaction: C + 2H.sub.2 .fwdarw.CH.sub.4. To produce methane from carbonaceous material according to this reaction, a source of hydrogen is required. This hydrogen is generally produced by the highly endothermic steam gasification reaction: C + H.sub.2 O .fwdarw.CO + H.sub.2. The steam gasification reaction creates a large thermal demand that must be met in any coal gasification process.
Many of the hydrogasification schemes proposed heretofore have focused on the conversion of carbon to methane in a fluidized solids reactor. In such a system, synthesis gas including hydrogen and carbon monoxide is passed upwardly through a dense turbulent bed of finely divided carbonaceous solids. The main source of heat for heat up of the fluidized carbonaceous solids in the fluid bed reactors has been sensible heat from the upflowing synthesis gas. See, e.g., U.S. Pat. Nos. 2,543,795; 2,609,283; 2,623,816; 2,662,816; 2,687,950; 2,694,623; 2,694,624 and 3,194,644. Fluid bed reactors are large and expensive pieces of equipment, however, and generally require long solids residence or holdup times, typically a minimum of about 10 minutes.
One difficulty in using coal as the carbon source for fluidized bed hydrogasification reactions is that devolatilization of the coal, particularly if it is a caking coal, tends to cause agglomeration of the fluidized particles and may interfere with fluidization. One suggested solution to this problem is to dilute the coal feed by premixing it with a stream of char circulated from the hydrogasification fluid bed reactor prior to introducing the coal into that reactor. See. e.g., U.S. Pat. Nos. 2,662,816 and 2,687,950. More often, however, it is proposed that the coal be subjected to a low temperature carbonization with air or oxygen to devolatilize and preheat it. This is done, for example, in the Hygas process developed by the Institute of Gas Technology.
In other instances, it has been proposed that coal devolatilization prior to introduction of the coal into a fluid bed steam gasification zone is effected by contacting the coal, either concurrently or countercurrently, with hot gases. As an example of a countercurrent contacting process, the Bureau of Mines Synthane process described in U.S. Pat. No. 3,463,623 calls for subjecting the coal to a pretreatment with steam and oxygen in a free fall zone surmounting and in open communication with a fluidized bed in which gasification occurs. As an example of a concurrent contacting, the Bituminous Coal Research Bi-Gas process, also known as the "two-stage, superpressure entrained" process, contemplates the introduction of freshly pulverized coal into an upper section of a gasifier and entrainment of the coal in a rising stream of hot synthesis gas produced in a slagging zone in the lower section of the gasifier. Heated by the synthesis gas, the cola devolatilizes and the residual char is swept out of the gasifier, where it is separated from the gas and recycled to the lower section of the gasifier. A similar system is shown in U.S. Pat. No. 2,713,590, where the feed coal is introduced into a high velocity effluent stream from a stream gasification zone shortly before the stream enters a gas-solids separator in order to permit devolatilization at relatively low temperature. Another example of concurrent contacting for coal devolatilization appears in U.S. Pat. No. 2,634,198, in which finely divided coal particles are conveyed by a nonoxidizing recycle gas through a heat transfer tube in a steam gasification reactor. Still another example of concurrent contacting of gases and solids is the process shown in U.S. Pat. No. 2,955,988, in which finely divided coal particles are conveyed under laminar flow conditions through a low temperature carbonization zone by means of a mixture of steam and a recycle gas from the carbonization zone.